The main functional task of a sensor is to produce an output signal representative of the detected amount or level of a measured physical phenomenon, of the type for which the sensor was pre-designed to detect. The produced output signal is then used by a wide variety of devices for different control or measurement purposes
Sensors will often generate output signals that have relatively low signal strength or a high noise component. Accordingly, it is known to provide such sensors with electronic circuitry for signal conditioning of the output signal so that the conditioned output signal can be more easily or more accurately sensed, interpreted or otherwise used by a control system. Such conditioning can involve but is not limited to processes such as amplification and/or filtering. For example, one simple and well-known solution for conditioning the output signal of a photodiode sensor is to use a linear trans-impedance operational amplifier. One example of such a linear trans-impedance operational amplifier is a Burr-Brown CMOS Rail-to-Rail IO Operational Amplifier OPA2357 sold by Texas Instruments, Dallas, Tex., USA.
However, in some situations, the use of such linear amplifiers may not be appropriate. Wide range sensors that theoretically generate a linear output signal over several decades may not be particularly compatible with such linear amplifiers. For example, photodiode sensors typically generate a response to light that is proportional to the amount of light received. Accordingly, when such sensors are used to detect daylight the amount of input varies widely from darkness to full daylight, and the sensor itself can detect and produce an output signal that is indicative of the sensed amount of light over a wide dynamic range. However, linear amplification is problematic because a tradeoff is typically made in such linear amplification circuits between the extent of the amplification (resolution) and operating signal range of the linear amplifier. That is, a linear amplifier that provides an appropriate level of resolution when the sensor output signal is configured to provide relatively low levels of output will typically be driven to provide an oversaturated response beyond which the linear amplifier provides no further resolution. Conversely, an amplifier that provides adequate resolution at the higher end of the sensor output will typically provide inadequate resolution for lower level output signals.
To solve this problem, logarithmic amplifiers can be used. One example of such a logarithmic amplifier is a Burr-Brown Precision Logarithmic and Log Ratio Amplifier, LOG104 also sold by Texas Instruments. As is noted in Texas Instruments' publication SBOS243C, published in May 2002, and revised in April 2005: “The LOG104 is a versatile integrated circuit that computes the logarithm or log ratio of an input current relative to a reference current. The LOG104 is operable over a wide dynamic range of input signals. In log ratio applications, a signal current can come from a photodiode, and a reference current from a resistor in series with a precision external reference.” Unfortunately, this solution is complicated and requires non-linear conversion of the output signal which provides a predetermined relationship which may not match the performance of a particular sensor.
Accordingly, what is needed in the art are a method and circuitry that enable wide dynamic range sensing having a linear output throughout the effective range of a sensor.